[X86][BF16] Customize INSERT_VECTOR_ELT for bf16 when feature BF16 is on

Fixes root cause of #63017.
The reason is similar to BUILD_VECTOR. We have legal vector type but
still soft promote for scalar type. So we need to customize these scalar
to vector nodes.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D155961